The multiple stage flashing of liquids has been most thoroughly developed in connection with the multistage flash (MSF) process for desalting of saline waters. This is the best accepted and most widely used desalination process and hence is most thoroughly covered in the patent literature. All known present-day MSF plants are basically horizontally arranged, so the brine can flow from one stage to the next simply as a result of the usual vapor pressure differences developed between stages. The flashing itself occurs simply in empty vessel space, unaided by mechanical means. An example of patents in this field is U.S. Pat. No. 2,944,599 (July 12, 1960, to A. Frankel), which illustrates the arrangement normally used. It consists simply of an opening at the bottom of the partition between two stages and an overflow weir in the downstream stage. Sometimes the stages are arranged at successively lower elevations (as U.S. Pat. No. 3,337,419, Aug. 2, 1967, to A. Kogan) to aid in brine transfer from one stage to the next. Others include adjustable weirs or gates to control the flow rate of brine between stages and hence avoid the risk of vapor bypassing from one stage to the next (as U.S. Pat. No. 3,172,824, Mar. 9, 1965, to S. L. Mulford). Mechanical means for conveying brine between stages are not now used in practice but are illustrated in U.S. Pat. No. 3,192,132 (June 29, 1965, to F. A. Loebel). This patent is similar to my present invention in that it shows conveying means (propellers) on a common shaft that serve to agitate the brine and promote flashing closer to equilibrium with the pressure in the vapor space. However, the said Leobel patent does not contemplate use of this device for conveying uphill or for flashing within the conveying device itself.
Little has been done to reduce the area required for flashing in these MSF evaporators. U.S. Pat. No. 3,197,387 (July 27, 1965, to H. R. Lawrence), for instance, shows only stationary means for spreading the flashing flow over a larger part of the vessel plan area in order to use this area more effectively. U.S. Pat. No. 3,214,349 (Oct. 26, 1965, to E. C. Keohoe et al.), is typical of a class of patents designed to reduce flash area requirements. However, these require downhill flashing through stationary devices arranged to more effectively separate the liquid from the flash vapor.
The uphill flashing of brine is not necessary in the normal MSF plant but is most desirable in the so-called vapor reheat process. Here, one descending stream must be heated or cooled countercurrently by an ascending stream, with the heat being transferred by flashing of one stream and direct condensation of the flashed vapor in the other stream. It usually matters little which stream moves uphill. Most concepts of this process employ pumps for transferring the stream moving uphill from one stage to the next (as U.S. Pat. No. 3,219,554, Nov. 23, 1965, to H. T. Woodward). Others, such as U.S. Pat. No. 3,249,517 to C. J. Lockman) show flashing of the hot stream uphill by vapor pressure differences and vapor-lift effect alone. Details of experimental work on this uphill flashing method are given by Newson, and Delve, Proc. Third Int'l. Symposium on Fresh Water from the Sea I, 127 (1970) and details of the processes in which the use of this flashing method is appropriate, including the standard MSF process, are given by Walker, Newson, and Johnson, Desalination 2, 96 (1967).
The multistage flash evaporator resulting from use of my invention possesses many novel flash devices exposed to successively lower pressures. Even though these units may be in direct communication with one another through their liquid connections, the pressure differences are maintained as an inherent function of the MSF evaporation cycle. Such maintenance will subsequently be shown in regard to FIGS. 1 and 5 herein.
In summary, although attempts have been made to move flashing brine uphill, to reduce the surface area required for flashing, and to achieve a closer approach to equilibrium between the flashing liquid and the flashed vapor and to flash foaming liquids, none have been able to do all these at one time whereas my device can do all or any of these in a positive and yet simple manner.
The flash devices of my invention are intended to provide a number of vapor streams at successively lower temperatures and pressures by exposing a liquid sequentially to successively lower pressures in each successive flash device. The means of maintaining these successively lower pressures are outside the scope of the invention but are well understood in the art. In brief, they involve condensing or absorbing the vapor by direct or indirect heat exchange contact with heat sinks at successively lower temperatures. The flash devices will be described primarily as used in a multistage flash evaporator since the means of maintaining these successively lower pressures here are most widely understood, having been extensively described in the literature and in patents, such as those listed above.